1. Field of the Invention
This invention relates to a process for separating indole in refined form, and more particularly to a process for separating indole in refined form from an indole-containing mixture by pressurized crystallization. Solid-liquid separation is carried out under pressure, yielding indole of high purity in solid form.
2. Prior Art
Indole is extremely important as a raw material for fragrances and flavors. Indoles is normally required to have high purity to function as a fragrance to a sufficient degree.
Heretofore, for the production of indole of high purity, it has been the usual practice to resort to a method (conventional method A) using coal tar or the like as a crude material, condensing the crude material, and then repeating the separation and refinement by a cooling crystallization process, ion-exchange process or adsorption process a method (conventional method B) forms indole into an alkali metal salt by an alkali fusion process or the like. After separating the metal salt, the separation is repeated with refinement by a cooling crystallization process or the like.
For instance, Japanese Laid-Open patent application No. 56-65868 proposes a method of separating indole from a fraction with a boiling point of 220.degree.-270.degree. C., which is obtained by distillation of coal. The indole is adsorbed and then desorbed by the use of anisole or the like.
Japanese Laid-Open patent application No. 57-98259 proposes a method of distilling crude indole, obtained from a coal tar fraction, until concentration of coexisting methylindole reaches a specific range, and producing refined indole by continuous cooling crystallization.
Described in Japanese Laid patent application No. 61-234789 is a method of adsorbing indole by passing an indole-containing tryptophane reaction liquid through a cation-exchange resin, and eluting indole with a hydrous organic solvent to obtain indole of high purity.
Further, an indole separating and refining method is described in "Newest Organic Chemistry 2", page 336, by Fezer, in which indole is formed into sodium salt by an alkali fusion process and then separated in refined form from the salt by a cooling crystallization process.
However, the above-mentioned conventional indole separating and refining methods have various problems as discussed below.
Namely, in the cooling crystallization process employed in the conventional method A, indole is crystallized out by cooling an indole-containing mixture, followed by solid-liquid separation. Therefore, it is necessary in the cooling crystallization stage to grow crystals which are suitable for the solid-liquid separation. That is to say, the cooling crystallization condition has to be strictly controlled. This gives rise to a problem that the operation for the cooling crystallization takes a long time. Besides, there is also a problem that scales are likely to deposit and its prevention makes the operation of cooling crystallization difficult. In addition, it is generally difficult to separate the crystal from the mother liquor, and this makes it difficult to obtain indole of high purity.
In the ion exchange process, a difficulty is encountered in separating indole form an eluent which is used for eluting indole adsorbed on an ion-exchange resin, and therefore the separation involves a complicated process. There is also a problem that regeneration or replacement of the ion-exchange resin further complicates the process.
The adsorption process has a problem in that the separation of indole from a zeolite desorbing agent is often difficult. Another problem is that regeneration or replacement of the adsorbent also complicates the process.
In the conventional method B, highly dangerous metallic sodium is used turn indole into an alkali metal salt by the alkali fusion process, wherein raising the temperature to induce the reaction involves safety problems. Despite the dangerous operation, the indole which is obtained after separation and refinement is low in purity, and needs a further refining operation.
Because of the above-discussed problems, the conventional indole separating and refining processes have not yet succeeded in realizing industrial production of high purity indole.